Atomic and molecular beam machines are powerful, widely used devices in the laboratory study of atomic and molecular properties, but they also find practical application in devices such as portable atomic frequency and time standards. In this latter application, they are integral parts of precision navigation and communications systems and frequently are used in highly dynamic or space environments. As the discussion throughout this application applies equally to most atomic and molecular materials from which one might form a beam, the two terms ("atom" and "molecule") will be used interchangeably throughout.
The on-axis flux in atomic beam ovens depends primarily on the source vapor pressure. Atomic beam ovens can be classified into three classes, referred to herein as dark-wall, bright wall and recirculating ovens, according to the manner in which the off-axis flux is controlled. Ideally, the collimation of the beam should involve simple geometric shadowing, that is, the collimator should just cut off the source emission in undesirable directions. However, it is difficult to achieve this end without introducing certain undesirable characteristics. For example, in a dark-wall oven for cesium atoms, a carbon collimator can be used to absorb every cesium atom which strikes it, thus achieving the desirable end, but the carbon soon saturates and the cesium deposited on the walls is either re-evaporated or, if it sticks, causes a change in the size of shape of the collimator. The dark-wall oven demonstrates a key problem in oven design, that is, dealing with the flux which strikes the walls of the collimator.
Conventional bright wall ovens use arrays of long narrow tubes to achieve good collimation. The array of narrow tubes allows for higher beam flux and for a good length-to-diameter (collimation) ratio in a short oven. To prevent these tubes from building up deposits of skimmed material, they are maintained at an elevated temperature and atoms which strike the wall are then re-evaporated with a cos (.theta.) distribution, wherein .theta. is the angle with respect to the normal to the source surface, as defined in Ramsey, N. F., Molecular Beams, Clarendon Press, Oxford (1956).
For a collimator of uniform cross section this process of absorption and re-emission of atoms leads to a vapor pressure which varies linearly between the pressure at the source and zero at the emitting end of the tube. This re-emission from the walls broadens the beam profile well beyond that produced by dark-wall ovens, but such bright-wall ovens have nonetheless proven to be very workable. If position along the tube is measured relative to the forward end, then the rate at which atoms are emitted from a wall-surface element at a distance z from the end of the tube is proportional to z. This assumption is not strictly valid at the front of the tube where an end correction should be made, but it appears to provide a good description of the central portion of the beam profile.
In a recirculating oven, wicking apparatus is provided to return collimated flux through capillary action for re-use by the source.
FIGS. 6 and 7 illustrate specific examples of well known prior art ovens. FIG. 6 shows the type of stable oven that would be used in a laboratory beam machine which does not need to operate over a long time period. The working material is contained in a heated chamber A and some of the vapors are allowed to escape through a small hole. The expanding cloud of vapor is intercepted by a collimater B which allows atoms with the correct trajectory to pass down the beam line. The total amount of material emitted through the oven hole can be shown to be: ##EQU1## where n is the number density of atoms or molecules in the oven chamber, v is their mean thermal velocity and A.sub.s is the area of the source hole. If the collimator hole can be characterized by a radius, r, separated from the oven hole by a distance, L, then the material emitted into the beam can be shown to be: ##EQU2## Thus, if L is very much larger than r, the effect of the collimator is to reduce substantially the total amount of material injected into the beam machine without affecting the on-axis beam flux.
The problem with this oven is the excessive amount of material which leaves the oven chamber but does not contribute to the beam. This material must be trapped behind the collimator. It cannot be allowed to find its way into the beam area or to plug the collimator.
The oven shown in FIG. 7 was developed in an attempt to deal with this problem. The working material is contained in a heated chamnber C and some of the vapors allowed to expand into a second chamber D at a slightly higher temperature. From here vapors pass through a multi-channel array E and into the beam chamber. The process of passing through the multi-channel array creates a quasi-collimated beam. The tubes of the collimator array are "bright wall" tubes, that is, any atom or molecule which strikes the wall of the tube must subsequently reevaporate and come back off the wall. Most of the atoms which enter a collimator tube return to the oven, while a smaller number travel the length of the tube and exit as part of the collimated beam. The effect of the "bright walled" tube collimator is to leave the forward directed flux unchanged, but to reduce the total amount of material leaving the oven to: ##EQU3## where r is the tube radius and L is its length.
While this device in part solves the excessive emission problem of the oven shown in FIG. 6, it suffers from several problems of its own. The collimation effect for a given aspect ratio (collimator hole area to length) has been reduced from ##EQU4## in the oven of FIG. 6, to ##EQU5## resulting in an increase in the amount of non-useful material injected into the beam area, material which can have long-term detrimental effects. The oven also requires anti-spill structures when used in a non-laboratory application, and with some materials, particularly those of interest to time standards, the small holes of the multichannel array have shown some tendency periodically to plug and unplug, giving rise to a spatially non-uniform and unstable beam.
A recirculating oven device which is considerably more complicated than the present invention is disclosed in R. D. Swenumson and U. Even, "Continuous Flow Reflux Oven as the Source of an Effusive Molecular Cs Beam," Rev. Sci. Instrum., 52(4): 559-561 (April 1981). This device uses a series of non-wicking baffles and collimators to provide the collimation effect, and a steel mesh to provide capillary action to return excess material caught by the baffles to the oven chamber. Its disadvantages include its complexity, its sensitivity to orientation and acceleration and the difficulty of reducing the size of the oven for commercial applications. In addition, its structure gives rise to condensate induced changes in beam shape and even plugging in the case of small source holes or the absence of gravity.